Introduction to Effective Permeability and Relative Permeability

1
Introduction toEffective PermeabilityandRelati
ve Permeability
2
Review Absolute Permeability

• Absolute permeability is the permeability of a
  porous medium saturated with a single fluid (e.g.
  Sw1)
• Absolute permeability can be calculated from the
  steady-state flow equation (1D, Linear Flow
  Darcy Units)

3
Multiphase Flow in Reservoirs

• Commonly, reservoirs contain 2 or 3 fluids
• Water-oil systems
• Oil-gas systems
• Water-gas systems
• Three phase systems (water, oil, and gas)
• To evaluate multiphase systems, must consider the
  effective and relative permeability

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Effective Permeability

• Effective permeability is a measure of the
  conductance of a porous medium for one fluid
  phase when the medium is saturated with more than
  one fluid.
• The porous medium can have a distinct and
  measurable conductance to each phase present in
  the medium
• Effective permeabilities (ko, kg, kw)

Amyx, Bass, and Whiting, 1960 PETE 311 Notes
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Effective Permeability
Steady state, 1D, linear flow equation (Darcy
units) qn volumetric flow rate for a specific
phase, n A flow area ?Fn flow potential drop
for phase, n (including pressure, gravity and
capillary pressure terms) ?n fluid viscosity
for phase n L flow length

• Oil
• Water
• Gas

Modified from NExT, 1999 Amyx, Bass, and
Whiting, 1960 PETE 311 NOTES
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Relative Permeability

• Relative Permeability is the ratio of the
  effective permeability of a fluid at a given
  saturation to some base permeability
• Base permeability is typically defined as
• absolute permeability, k
• air permeability, kair
• effective permeability to non-wetting phase at
  irreducible wetting phase saturation e.g.
  ko(SwSwi)
• because definition of base permeability varies,
  the definition used must always be
• confirmed before applying relative permeability
  data
• noted along with tables and figures presenting
  relative permeability data

Amyx, Bass, and Whiting, 1960
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Relative Permeability

• Oil
• Water
• Gas

So 0.5
Sw 0.3
?Sg 0.2
Modified from Amyx, Bass, and Whiting, 1960
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Relative Permeability Functions
Imbibition Relative Permeability
1.00
kro _at_ Swi

• Wettability and direction of saturation change
  must be considered
• drainage
• imbibition
• Base used to normalize this
• relative permeability curve is
• kro _at_ Swi
• As Sw increases, kro decreases
• and krw increases until
• reaching residual oil
• saturation

0.80
Two-Phase Flow Region
Residual Oil Saturation
0.60
Relative Permeability (fraction)
Irreducible Water Saturation
Oil
0.40
0.20
krw _at_ Sor
Water
0
0.40
0
1.00
0.60
0.20
0.80
Modified from NExT, 1999
Water Saturation (fraction)
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Effect of Wettabilityfor Increasing Sw
1.0
0.8
0.6
Relative Permeability, Fraction
Oil
0.4
0.2
Water
0
40
0
100
60
20
80
Water Saturation ( PV)
Strongly Water-Wet Rock

• Water flows more freely
• Higher residual oil saturation

Modified from NExT, 1999
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Factors Affecting Relative Permeabilities

• Fluid saturations
• Geometry of the pore spaces and pore size
  distribution
• Wettability
• Fluid saturation history (i.e., imbibition or
  drainage)

After Standing, 1975
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Characteristics of Relative Permeability Functions

• Relative permeability is unique for different
  rocks and fluids
• Relative permeability affects the flow
  characteristics of reservoir fluids.
• Relative permeability affects the recovery
  efficiency of oil and/or gas.

Modified from NExT, 1999
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Applications ofRelative Permeability Functions

• Reservoir simulation
• Flow calculations that involve multi-phase flow
  in reservoirs
• Estimation of residual oil (and/or gas)
  saturation